1. Field of Invention
The invention relates to a case for an automatic transmission and, in particular, to a vibration resistant structure thereof.
2. Description of Related Art
A case for an automatic transmission is generally formed from a transmission casing that covers the transmission mechanism, a transmission apparatus housing that is fastened to the front of the casing and that covers a hydraulic power transmission apparatus comprising a torque converter or the like, and an extension housing that is mounted to the rear of the transmission casing. The valve body of the hydraulic control apparatus that controls the hydraulic power transmission apparatus and the transmission mechanism of the automatic transmission is usually mounted on the side of the transmission casing. In an automatic transmission for a front enginexe2x80x94rear drive (FR) vehicle, in particular, the valve body is normally fixed to the bottom of the transmission casing and protrudes downwards from the casing. The valve body is completely enclosed by an oil pan that covers the area below and surrounding the valve body. A representative example of an automatic transmission case having this type of structure is disclosed in Japanese Patent Laid-Open No. HEI 7-259967.
Because the size of the outer diameter is severely restricted in an automatic transmission for an FR vehicle in order to ensure sufficient space in the vehicle interior, as is not the case with an automatic transmission for a front engine front drive (FF) vehicle, an automatic transmission for an FR vehicle has a narrow elongated shape in which the size of the outer diameter is small in comparison with the size in the axial direction due to each element used to change gears being aligned in the axial direction. Therefore, it is difficult to secure rigidity in the automatic transmission and when this is mounted in a vehicle and is regarded together with the engine as a vibration system, if the rigidity of this vibration system is low, the vibration system has a resonance point in a comparatively low frequency range near the vibration frequency of the propeller shaft. Accordingly, if this resonance frequency is a frequency that causes a harsh grating sound to be heard in the audible frequency band, then it is not possible to achieve the level of quietness expected in a running vehicle.
In order to reduce the noise generated from the transmission due to the resonance and ensure an appropriate level of quietness, it is necessary to eliminate the resonance and vibration of the propeller shaft by providing the transmission with a characteristic vibration frequency in a higher frequency region. To achieve this, it is necessary to increase the rigidity. In particular, when a drive apparatus for a four wheel drive vehicle is formed using an automatic transmission for an FR vehicle as the base thereof, in terms of mass, because a transfer having only a slightly smaller mass than the transmission is linked to the rear side of the transmission, it is easy for a vibration system having a long axial length and having a low rigidity joint to be formed between the two large mass portions that are linked to each other. Thus, it is difficult to reduce the noise further without increasing the rigidity of the transmission case to which the transfer is linked.
Making the case thicker is one simple means of improving the rigidity of a transmission. However, if the case is made thicker, the mass of the case itself is also thereby increased resulting instead in a vibration system with a linking portion having a relatively low rigidity being formed and the desired effect of an improvement in the level of quietness is not obtained.
Therefore, it is the main aim of the invention to provide a transmission case capable of providing increased rigidity while holding the increase in the mass of the transmission to the absolute minimum in order to ensure that the transmission operates quietly.
Accordingly, it is an object of the invention to provide an automatic transmission case in which a valve body is attached to a case wall covering a transmission mechanism, wherein an enclosing wall that projects outwards from the case wall enclosing the periphery of the valve body is provided in the case.
It is effective if the enclosing wall in the above structure is formed higher than the thickness of the valve body, and the opening of the space for containing the valve body that is enclosed by the enclosing wall is covered by an oil pan that is shallower than the thickness of the valve body.
Moreover, in a structure in which the case comprises a transmission casing that covers the transmission mechanism and a transmission apparatus housing that covers a hydraulic power transmission apparatus, it is also effective if the transmission apparatus housing has a joining portion where it is joined to the transmission casing, the enclosing wall is provided in the transmission casing, and the front portion of the enclosing wall forms a matching portion that matches the joining portion of the transmission apparatus housing; and the transmission apparatus housing is mounted to the transmission casing by matching the joining portion to the front portion of the enclosing wall and fastening the joining portion to the front portion of the enclosing wall using fastening means.
In this case, it is effective if the joining portion is formed as a joining wall that faces the front portion of the enclosing wall of the transmission casing, and the joining wall is provided with ribs that connect the rear of the joining wall to the transmission apparatus housing.
Furthermore, it is effective if the valve body is fastened by fastening means to the case wall in the vicinity of the front portion and the rear portion of the enclosing wall.
Moreover, it is also effective if the rear wall is provided in the case wall in the vicinity of a supporting portion that supports an output shaft of the transmission mechanism.
Furthermore, it is also effective if the rear wall is provided with ribs that connect the rear of the rear wall with the rear end portion of the transmission casing.
Moreover, it is also effective if the case comprises a transmission casing that covers the transmission mechanism and a transmission apparatus housing that covers a hydraulic power transmission apparatus, and a rib is provided extending from the front end to the rear end of the transmission casing that is integral with a portion of the case wall other than the portion of the transmission casing where the valve body is mounted.
According to an aspect of the invention, because the space needed for attaching the valve body is used to enable a case structure to be formed in which the geometrical moment of inertia of the casing is enlarged by forming the enclosing wall that encloses the periphery of the valve body attached to the transmission case projecting out from the transmission case, the rigidity of the case is improved while holding any increase in the mass to a minimum, without causing any increase in the mass to be generated as a result of increasing the thickness of the case wall itself in order to improve the rigidity.
In a preferred form of the invention, because the space to contain the valve body can be secured using the shallow oil pan that covers the opening of the containing space and using the enclosing wall that is higher than the thickness of the valve body, there is no need to lower the position of the oil pan because the enclosing wall has been made to project out in order to ensure sufficient rigidity, and the minimum ground clearance when the automatic transmission is mounted in a vehicle can be secured in the same way as in a conventional structure.
In a preferred form of the invention, when the transmission case is formed from a transmission casing that covers the transmission mechanism and a transmission apparatus housing that covers a hydraulic power transmission apparatus in the same way as in a conventional structure, because the transmission casing and the power transmission housing are linked directly by the matching portion formed by the front portion of the enclosing wall on the casing side and the joining portion of the housing, any reduction in the rigidity of the linking portion due to a separate transmission casing being linked to a power transmission housing is controlled.
In a preferred form of the invention, ribs at the rear of the joining wall of the transmission apparatus housing fastened to the front portion of the enclosing wall of the transmission casing have the function of improving the rigidity of the transmission apparatus housing and, as a result, the rigidity of the entire transmission case including the rigidity of the transmission casing linked to the transmission apparatus housing is improved.
In a preferred form of the invention, because the valve body is fastened by fastening means to the case wall in the vicinity of the front portion and the rear portion of the enclosing wall that encloses the valve body, the rigidity of the valve body, which conventionally has had a high degree of rigidity, can be used to improve the rigidity of the transmission casing.
In a preferred form of the invention, by providing the rear wall of the enclosing wall in the vicinity of the case wall supporting the output shaft, the enclosing wall also helps to improve the rigidity of the output shaft supporting portion of the case wall and the rigidity of the output shaft support is also improved
In a preferred form of the invention, because it is possible to improve the rigidity of the rear portion of the casing using the rigidity of the enclosing wall, the result is that the rigidity of the transmission casing can be improved even further.
In a preferred form of the invention, it is possible to improve the rigidity of the overall transmission casing by providing a balanced rigidity in response to the improved rigidity of the portion to which the valve body is attached.